I currently use an ipad 4, with its 10" screen, as a second screen for my mpb retina 13. I like this setup because it's thin and relatively light so slots easily into my bag along with my macbook, and doesn't require a power socket to run, just USB. I also use the ipad for reading and a few other tasks so carrying it around has intrinsic value in addition to being a second screen.
The ipad 4 does a reasonable job, but it's a bit on the small side. This new pro form factor is, however, perfect - 12.9" and higher res than my mbp retina screen means it really can be a true second screen, with roughly identical capabilities to my main screen. I'm pretty happy about this.
Won't be upgrading just yet on account of the price, but when it drops enough, replacing my current ipad with this will be a no-brainer.